1. Field of the Invention
The present invention relates to an X-ray apparatus including a rotatable table for rotating a subject and being capable of operating in a first imaging mode of performing X-ray imaging while the subject is rotated and a second imaging mode of performing X-ray imaging while the subject remains stationary.
2. Description of the Related Art
A cone-beam X-ray computerized tomography (CT) apparatus is schematically shown in FIG. 2A. In this apparatus, a rotatable table 1 is disposed between an X-ray tube 103 and a flat-panel sensor 101. A chair 2 is fixed on the rotatable table 1.
As shown in FIG. 2B, X-ray imaging is performed on a subject 102 sitting on the chair 2 while the rotatable table 1 is rotated. X-rays emitted from the X-ray tube 103 are absorbed and attenuated in the body of the subject 102. The transmitted X-rays are then detected by the flat-panel sensor 101. The subject 102 is rotated about its axis while a positional relationship between the X-ray tube 103 and the flat-panel sensor 101 is maintained, so that projected image data in one turn is obtained. The obtained projected image data is reconstructed, so that a tomographic image is obtained. Instead of rotating the subject by one turn, the X-ray tube 103 and the flat-panel sensor 101 may be moved.
Cone-beam CT apparatuses are disclosed in, for example, Japanese Patent Laid-Open Nos. 52-140286 (corresponding to U.S. Pat. No. 4,173,720) and 60-207041. Apparatuses for performing plain imaging using flat-panel sensors are disclosed in, for example, Japanese Patent Laid-Open No. 10-201745. Techniques for withdrawing a movable top board from an X-ray irradiation field in an apparatus using a flat-panel sensor are disclosed in, for example, Japanese Patent Laid-Open Nos. 11-137544 (corresponding to U.S. Pat. No. 6,152,598) and 2003-250787.
A flat-panel sensor is relatively expensive, so the purchase of multiple flat-panel sensors entails high cost. Therefore, it is desire that a single cone-beam CT apparatus perform both cone-beam CT imaging and usual radiography imaging (usual imaging). However, a conventional cone-beam CT apparatus cannot realize sufficient definition when performing the usual imaging because the chair 2, including a seat 3 and a backrest 4 and disposed on the rotatable table 1, is imaged in the resulting image.
For example, when the subject is human, as shown in FIG. 2B, the chair 2 is included in the irradiation field, and therefore, the chair 2 is imaged as a projected image in the resulting image. On the other hand, when performing the cone-beam CT imaging on, for example, a knee, the chair 2 on the rotation axis will be an obstacle because the knee must be placed at the rotation center.